Ceramic ware plate useful for materials processing equipment

ABSTRACT

An improved ware plate useful for materials feeder equipment, and the like, includes highly efficient, durable and wear and abrasion resistant sliding surface for slidably contacting elements of the feeder equipment. In particular, the ware plate has a tetragonal zirconia polycrystal ceramic sliding surface for contacting an element of the feeder equipment having a zirconia-alumina ceramic composite sliding surface. Alternatively, the sliding surface of the ware plate is zirconia-alumina ceramic composite for sliding contact with the element of the feeder equipment having a tetragonal zirconia polycrystal ceramic sliding surface.

TECHNICAL FIELD

This invention relates generally to an improved ware plate. Moreparticularly the invention concerns an improved ware plate useful formaterials processing equipment, such as powder feeder equipment, inwhich one or more elements are in sliding contact therewith. The wareplate has a sliding surface comprising selectively a high precision,durable, wear resistant tetragonal zirconia polycrystal (TZP) ceramic ora zirconia-alumina ceramic composite.

BACKGROUND OF THE INVENTION

Material feeder devices are widely used in many industries to transportparticles such as powders, pellets, medicaments, metals, and the like.In compression molding operations, efficient filling of the molds withmaterials in the powder form as a significant bearing in determining theunit manufacturing cost (UMC). Feeder boxes are used to carry loosepowder to the top of the die where it shakes dropping powder into thedie cavity. The die cavity is located underneath the die clamp ring orthe die retaining ring. The die cavity is filled by the shaking motionof the feeder box which is connected to a hydraulic actuated cylinder.The feeder box slides on a flat surface called "ware plate" in areciprocating motion. The time interval between the two extremelocations of the feeder box, when the compacting of the powders in thedie cavity is effected. The top surfaces of the ware plate, the dieretaining ring and the bottom surface of the feeder box need to be atthe same level for the smooth operation of the powder filling mechanism.When the die cavity is filled with powder, it is compressed by theaction of punches making a "green part". The proper filling of the diecavity depends, among several other factors, on the sliding motion ofthe bottom of the feeder box on a plate normally known as feeder boxware plate and also on the die clamp (or retaining) ring. If there is amisalignment of the feeder box with respect to the ware plate and alsowith respect to the die clamp (or retaining) ring or if there is aslight gap between those components, excessive powder loss results. Thisproblem is magnified if the powder particles are very fine (submicron)or are very hard. In such cases, the jamming of the feeder box (in worstcase, breakage of the feeder box or ware plate or die clamp ring) canlead to interruption of the manufacturing process and a higher UMC forthe part.

Misalignment of the feeder box with respect to the ware plate and/orwith respect to die clamp ring can occur due to gouging of one of thesurfaces, which is a common occurrence in industrial compression moldingmachines where the box and the plate and also the clamp ring are usuallymade of steels. Repeated sliding of two surfaces of metal parts, as inthis specific case of surfaces of feeder box and the ware plate (or dieclamp ring), usually leads to excessive wear and abrasion of thosesurfaces leading to the loss of materials, contaminating the powderfeed, and also creating a gap between the box and the plate and alsobetween the feeder box and the die clamp ring. This gap between thefeeder box and the ware plate and also between the feeder box and thedie clamp ring leads to the loss of powder, and in some cases jamming ofthe feeder mechanism. Normally, the machine components will begin toshow signs of wear at about 5000 cycles of operation. At about onemillion cycle, the feeder plate will have to be replaced, if not sooner.Powdered material will start leaking out from under the feeder box andfalling on the rest of the movable components of the machine makingmaintenance a large problem. Also, the motion of the feeder box willbecome agitated and impede the free flowing motion of powder during diefilling.

In order to overcome the aforementioned problems one needs to search forthe right material. Experience indicates that ytrria-doped tetragonalzirconia polycrystal (Y-TZP) ceramic materials offer many advantagesover conventional materials, including many other ceramics. Y-TZP is oneof the toughest ceramics. The toughness is achieved at the expense ofhardness and strength. Tetragonal zirconia alloy-alumina composite, thatis, the product of sintering a particulate mixture of zirconia alloy andalumina, is another tough and relatively soft structural ceramiccomposite. Y-TZP ceramic and zirconia-alumina composites havetribological properties that are not as attractive as other highperformance structural ceramics like SiC and Si₃ N₄. However, most ofthe conventional high performance ceramics are extremely brittle. Anexample of a material having good hardness and strength is monolithiccubic spinel, however, this material is also highly brittle and isunusable for structural applications.

It is further known that a powder feed assemblage having a feeder boxsurface, particularly the lower surface of the box in sliding contactwith a ware plate surface and die clamp ring has a longer service lifeand better performance if made with a relatively hard material havinghigh fracture toughness and the mating surfaces have low coefficientfriction.

An alternative approach is taught by U.S. Pat. No. 5,358,913, which ishereby incorporated herein by reference. In that approach, a tetragonalzirconia alloy article, which can be near net-shape, is compacted andthen sintered in the presence of an MgO, CaO, Y₂ O₃, Sc₂ O₃, Ce₂ O₃, orother rare earth oxide dopant to produce an article having a tetragonalcore and a cubic case. The dopant can be provided in a number ofdifferent forms such as a solid plate, a powder, or a layer produced bydecomposition of an organo-metallic precursor film. In U.S. patentapplication Ser. No. 07/994,820, now abandoned in favor ofcontinuation-in-part application Ser. No. 08/231,870, filed Apr. 25,1994, now U.S. Pat. No. 5,677,072 a method is described for producingarticles having a tetragonal zirconia alloy core and a monoclinic case.In U.S. patent application Ser. No. 07/994,818, now abandoned in favorof a Continuation-in-Part Application U.S. Ser. No. 08/400,416, nowabandoned hereby incorporated by reference, a method is described forproducing articles having a tetragonal zirconia alloy and α-alumina(alpha-Al₂ O₃) core and a case of tetragonal zirconia and cubic spinel.In the core and the case the predominant species is tetragonal zirconia.The application also teaches a method for producing articles having acore that is tetragonal zirconia alloy along with less than about 5weight percent alumina and having a case that is cubic phase zirconiaand cubic spinel (MgAl₂ O₄). The α-alumina is about as hard as cubiczirconia. These types of ceramics with composite structures with varyingdegrees of physical and mechanical properties are termed as"functionally gradient ceramics". In functionally gradient ceramics,where the core and shell of the ceramic bodies having different crystalstructures hereby disclosed in above-referenced U.S. Patents and U.S.Patent Applications.

As will be more completely disclosed, the method of our inventionapplies to a ware plate useful for powder feeder assemblages.

Therefore, a need persists for an improved ceramic ware plate method ofmaking same such that the ware plate has superior wear and abrasionresistance while being cost effective and easy to manufacture.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide animproved, high precision ceramic ware plate useful for materials feederequipment that is reliable, simple to use and cost effective tomanufacture.

It is another object of the invention to provide high precision ceramicware plate useful for materials feeder equipment in which contactingsurfaces have remarkably improved wear and abrasion resistance, andtherefore, a longer useful life.

It is a feature of the invention that the improved ceramic ware plateuseful for materials feeder equipment alternatively has either atetragonal zirconia ceramic material (Y-TZP) sliding surface or azirconia-alumina ceramic sliding surface for sliding contact with otherelements of the materials feeder equipment.

Accordingly, for accomplishing these and other objects, features andadvantages of the invention, there is provided, a ware plate useful formaterials feeder equipment in which at least one element has a slidingsurface comprising tetragonal zirconium polycrystal ceramic for slidablycontacting the ware plate, wherein the ware plate compriseszirconia-alumina ceramic composite.

In another aspect of the invention, a ware plate useful for materialsfeeder equipment in which at least one element has a sliding surfacecomprising zirconium-alumina ceramic composite for slidably contactingthe ware plate, wherein the ware plate comprises tetragonal zirconiapolycrystal ceramic.

In another aspect of the invention, a method of making a precisionceramic ware plate useful for powder feed equipment which includes thestep of providing ceramic powder comprising either a solely of zirconiaalloy or a composite comprising of first concentration of particulatezirconium oxide alloy and a second concentration of particulate aluminumoxide. The zirconium oxide alloy consists essentially of zirconium oxideand a secondary oxide selected from the group consisting of MgO, CaO, Y₂O₃, Sc₂ O₃, and rare earth oxides. Moreover, wherein zirconium oxidealloy has a concentration of said secondary oxide of, in the case of Y₂O₃, about 0.5 to about 5 mole percent; in the case of MgO, about 0.1 toabout 1.0 mole percent, in the case of CeO₂, about 0.5 to about 15 molepercent, in the case of Sc₂ O₃, about 0.5 to about 7.0 mole percent andin the case of CaO from about 0.5 to about 5 mole percent, relative tothe total of said zirconium oxide alloy, said compacting furthercomprising forming a blank. A mold is provided for receiving andprocessing the ceramic powder. The ceramic powder is then compacted inthe mold provided to form a ceramic green preform or preform. Theceramic preform is then shaped or machined so as to form a nearnet-shaped ware plate. In this embodiment of the invention, after theinitial shaping, the near net-shaped green ceramic ware plate issintered thereby forming a sintered ceramic ware plate. The ceramic wareplate is then further machined or shaped. Alternatively, the net-shapecomponents of the feeder assemblage are made either by injection moldingor by dry pressing. Net-shape manufacturing of ceramic articles,particularly Y-TZP articles are disclosed in U.S. Pat. No. 5,336,282which is hereby incorporated herein by reference.

It is, therefore, an advantage of the invention that the method formaking a ceramic ware plate useful for powder feeder equipment, and thelike, is reliable, easy to use, cost effective and efficient topractice. Moreover, ware plates made with the method of the inventionimparts low cost to the product, while having characteristically highreliability, a longer life, easier manufacturability, and improved wearand abrasion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other objects, features and advantages of theinvention and the manner of attaining them will become more apparent andthe invention itself will be better understood by reference to thefollowing description of an embodiment of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a perspective view of the materials feeder equipment of theinvention;

FIG. 2 is a top plan view of the materials feeder equipment of theinvention;

FIG. 3 is a side view of the materials feeder equipment showing thefeeder box in the first position along the ware plate;

FIG. 4 is a side view of the materials feeder equipment of the inventionshowing the feeder box in a second position on the ware plate;

FIG. 5 is a schematic flow diagram of the method of the invention; and

FIG. 6 is a schematic of a wet bag isostatic pressing machine useful inthe method of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To overcome the aforementioned problems in the prior art, we carefullymanufactured and investigated special ceramic materials for use in theware plate useful in, for instance, materials feeder equipment 100 shownin FIG. 1. After considerable investigation and surprise we found thatytrria-doped tetragonal zirconia polycrystal (Y-TZP) ceramic materialsoffer many advantages over conventional materials, including many otherceramics. Y-TZP is one of the toughest ceramics. The toughness isachieved at the expense of hardness and strength. Tetragonal zirconiaalloy-alumina composite, that is, the product of sintering a particulatemixture of zirconia alloy and alumina, is another tough and relativelysofter structural ceramic composite.

In FIGS. 1-4, an improved materials feeder equipment 100 according tothe principles of the invention is illustrated. Materials feederequipment 100 may have a dispenser, such as a hopper (not shown), forrespectively receiving and then dispensing materials. A feeder box 114is provided having means, such as an opening, to receive materials fromthe dispenser and a second opening (not shown) for controllablyreleasing the materials from the feeder box 114. Importantly, feeder box114 has a base, or alternately a first sliding portion, 120, comprisingeither tetragonal zirconia polycrystal ceramic or zirconia-aluminaceramic composite. Base 120 further has sidewalls 124 surrounding base120 for containing the materials therein. Moreover, materials feeder 100comprises a ware plate 126 which slidably supports feeder box 114. It isimportant to the invention that ware plate 126 includes a second slidingportion 128 formed at least partially of either a zirconia-aluminaceramic composite or tetragonal zirconia ceramic, as described indetails below. Second sliding portion 128 of the ware plate 126 isarranged for sliding contact with the first sliding portion 120 of thebase 120 of feeder box 114. Preferably, ware plate 126 is stationary andthe first sliding portion 120 of feeder box 114 is slidable along secondsliding portion 128 of ware plate 126.

Means, such as a pneumatically controlled ram 130 is used slidablymoving the first sliding portion 120 of the feeder box 114 along thesecond sliding portion 128 of the ware plate 126. Feeder box 114 isconfigured to move between a first position where loose powder is fedinto the feeder box 114 through, for instance a hopper (not shown), anda second position where the loose powder is released from the feeder box114 into a die cavity 132, described below.

In an alternative embodiment of the invention, a die cavity 132 may bearranged in the ware plate 126 for receiving powder released by thefeeder box 114 when said feeder box 114 is in the second position, asdescribed above. A die retaining element, or alternatively a dieretaining ring, 134, is arranged in the die cavity 132 to secure the diecavity 132 during materials processing. Thus, die retaining ring 134comes into sliding contact with the first sliding portion 120 of feederbox 114. Moreover, die retaining ring 134 is arranged in a continuousplane with the ware plate 126 so that feeder box 114 can travel smoothlybetween the first and second positions. Finally, a die punch 136 isarranged above the ware plate 126 and the second position of the feederbox 114 for accessing the die cavity 132. Thus the die punch 136 cancompress the powder released into the die cavity 132 thereby forming thedesired molded part.

In a preferred embodiment of the materials feeder equipment 100 of theinvention, second sliding portion 128 of ware plate 126 iszirconia-alumina ceramic composite; and, first sliding portion 120 offeeder box 114 is tetragonal zirconia ceramic.

In an alternate embodiment of the invention, materials feeder equipment100 has second sliding portion 128 of ware plate 126 comprisingtetragonal zirconia ceramic; and, the first sliding portion 120 offeeder box 114 comprises zirconia-alumina ceramic composite.

In yet another embodiment of the invention, at least one catch bin maybe movably positioned for alignment with a cavity in the ware plate atthe second position. Each catch bin, in this embodiment, has a ceramicsurface which is alignable in a continuous plane with the ware plate forslidably contacting the base of the feeder box. The ceramic surface ofcatch bin would comprise either tetragonal zirconia ceramic orzirconia-alumina ceramic composite, corresponding to the second slidingportion of the ware plate.

The method of making the improved materials feeder equipment 100 havingceramic elements (described above) is broadly illustrated in FIG. 5(Steps A to F). According to the method, a ceramic powder comprising ofprimarily zirconium oxide alloy, or a mixture of particulate zirconiumoxide alloy and a second concentration of aluminum oxide is used formaking materials feeder equipment 100, as discussed fully below.Elements of the materials feeder equipment 100 made in accordance withthe method of the invention is illustrated in FIG. 5 (Step F) and FIG.6. The zirconium oxide alloy consists essentially of zirconium oxide anda secondary oxide selected from the group consisting of MgO, CaO, Y₂ O₃,Sc₂ O₃, and rare earth oxides. Moreover, the zirconium oxide alloy has aconcentration of the secondary oxide of, in the case of Y₂ O₃, about 0.5to about 5 mole percent; in the case of MgO, about 0.1 to about 1.0 molepercent, in the case of CeO₂, about 0.5 to about 15 mole percent, in thecase of Sc₂ O₃, about 0.5 to about 7.0 mole percent and in the case ofCaO from about 0.5 to about 5 mole percent, relative to the total ofsaid zirconium oxide alloy, said compacting further comprising forming ablank. A mold is provided for receiving and processing the ceramicpowder. The ceramic powder is then compacted (as described below) in themold provided to form a ceramic preform. The ceramic preform is thenshaped or green-machined so as to form independently near net-shapedgreen molding elements, i.e., first and second sliding portions offeeder box and a ware plate, respectively. In this embodiment of theinvention, after the initial shaping, the green ceramic moldingcomponents are sintered thereby forming a sintered net-shape ceramicmolding components, as described more fully below. The ceramic elementsfor the materials feeder equipment 100, described above, are thenfurther machine or shaped until finished components are formed.Subsequently, the ceramic elements are arranged in the materials feederequipment 100 as described above.

More particularly, the method of making the feeder equipment elements,including the first and second sliding portions of the feeder box andware plate, respectively, useful for the improved materials feederequipment 100 of the invention, are set forth in details below.

Ceramic Powder Material Mixing

According to FIG. 5, step A diagrammatically illustrates the alloyingprocess. Zirconia powder 140 is combined with one or more secondaryoxide powders 142 to provide zirconia alloy powder 144. The preparationof zirconia alloys is well known to those skilled in the art andzirconia alloys are available commercially. For example, particulatezirconia alloy having 3 mole percent Y₂ O₃ is marketed by Z-TECHCorporation, Bow, N.H., as "SYP-ULTRA 5.2 Yttria Stabilized Zirconia"(presently, HANWHA Advanced Ceramics, as "HWA-3YB") or by TOSOHCorporation of Japan, as "TZ-3YB".

More particularly, we prefer using tetragonal zirconia ceramic materialfor manufacturing components of the ceramic materials feeder 100 in acost effective way. The most preferred material which we prefer using isessentially zirconia having 100 percent tetragonal crystal structure. Wedeveloped this 100 percent tetragonal zirconia by alloying zirconia witha number of secondary oxides as described in U.S. Pat. Nos. 5,336,282and 5,358,913, hereby incorporated herein by reference.

The preferred ceramic composite powder mixture most preferred in themethod of making zirconia-alumina composites of the invention includes aparticulate zirconia alloy and a particulate alumina made by mixing ZrO₂and additional "secondary oxide" selected from: MgO, CaO, Y₂ O₃, Sc₂ O₃and Ce₂ O₃ and other rare earth oxides (also referred to herein as"Mg-Ca-Y-Sc-rare earth oxides") and then with Al₂ O₃. Zirconia alloysuseful in the methods of the invention have a metastable tetragonalcrystal structure in the temperature and pressure ranges at which theceramic article produced will be used. For example, at temperatures upto about 200° C. and pressures up to about 1000 Mpa, zirconia alloyshaving, wherein zirconium oxide alloy has a concentration of saidsecondary oxide of, in the case of Y₂ O₃, about 0.5 to about 5 molepercent; in the case of MgO, about 0.1 to about 1.0 mole percent, in thecase of Ce₂ O₃, about 0.5 to about 15 mole percent, in the case of Sc₂O₃, about 0.5 to about 7.0 mole percent and in the case of CaO fromabout 0.5 to about 5 mole percent, relative to the total of saidzirconium oxide alloy, said compacting further comprising forming ablank and then sintering, exhibit a tetragonal structure. Preferredoxides for alloying with zirconia are Y₂ O₃, MgO, CaO, Ce₂ O₃ andcombinations of these oxides. It is preferred that the zirconia powderhave high purity, greater than about 99.9 percent. Specific examples ofuseful zirconia alloys include: tetragonal structure zirconia alloyshaving from about 2 to about 5 mole percent Y₂ O₃, or more preferablyabout 3 mole percent Y₂ O₃. Examples of tetragonal structure zirconiaalloys useful in the methods of the invention are disclosed in U.S. Pat.No. 5,290,332. Such zirconia alloys are described in that patent asbeing useful to provide a "net-shape" ceramic article: a ceramic articlethat is dimensionally true after sintering and, therefore, does notnecessitate further machining prior to use in its intended workingenvironment.

Referring again to FIG. 5, Step A' diagrammatically illustrates analternative mixture of particulate zirconia alloy powder 144 and aparticulate aluminum oxide 146. This alternative mixture can be achievedby mixing mechanically or chemically, for example, mixing byco-precipitation. The particulate mixture formed is from about 50 to 100percent by weight (weight/total weight of particulate mixture) ZrO₂, andpreferable is from about 80 to about 99 percent by weight ZrO₂, or morepreferably is from about 80 to about 95 percent by weigh ZrO₂ and thebalance being Al₂ O₃. The product of this alternative mixture iszirconia-alumina ceramic composite 148.

The grain and agglomeration sizes and distributions, moisture contents,and binders (if any) can be varied in both the alumina and the zirconiaalloy, in a manner known to those skilled in the art. "Grain" is definedas an individual crystal, which may be within a particle, having aspatial orientation that is distinct from that of adjacent grains."Agglomerate" is defined as an aggregation of individual particles, eachof which may comprise multiple grains. In a particular embodiment of theinvention, the grain and agglomeration sizes and distributions, andmoisture contents of the alumina and the zirconia alloy aresubstantially the same and are selected as if the zirconia alloy was notgoing to be mixed with the alumina, that is in a manner known to the artto be suitable for the preparation of a zirconia alloy article.

An example of convenient particulate characteristics for a particularembodiment of the invention is the following. Purity of ZrO₂ ispreferably well controlled at 99.9 to 99.99 percent, that is, impuritiesare no more than about 0.1 to 0.01 percent. The grain size is from about0.1 micrometers to about 0.6 micrometers. The average grain size is 0.3micrometers. The distribution of grain sizes is: 5-15 percent less than0.1 micrometers, 40-60 percent less than 0.3 micrometers, and 85-95percent less than 0.6 micrometers. The surface area of each individualgrain ranges from about 10 to about 15 m² /gram or is preferably 14 m²/gram. Agglomerate size is from about 30 to about 60 micrometers andaverage agglomerate size is: 40-60 micrometers. Moisture content isabout 0.2 to 1.0 percent by volume of blank and is preferably 0.5percent. The mixture of particulate is compacted in the presence of anorganic binder.

Referring once again to FIG. 5, Step B, binders such as gelatin orpolyethylene glycol(PEG) or acrylic or polyvinyl ionomer or morepreferably polyvinyl alcohol, are added to and mixed with theparticulate mixture Y-TZP, 144 or a composite mixture of Y-TZP andalumina 148, both illustrated in Step A and A' respectively. This can beachieved preferably by spray drying or ball milling prior to placementof the mixture in a compacting device.

Further, Step B also illustrates an alternative mixing process known tothose who are proficient in the art as "compounding" in which theparticulate mixture or mixtures are mixed with greater than 20 percentby weight of an organic binder such as paraffin at a temperature higherthan the glass transition temperature of such binder for subsequentinjection molding process, illustrated as Step C'.

Compacting

Turning now to compacting and more particularly to the processillustrated in FIG. 5, the particulate mixture either 144 or 148 is coldcompacted using preferably an isostatic press 153 to provide anunsintered blank 152 in Step C (FIG. 5). Unsintered blank 152 isalternatively referred to herein as a "green preform". It should beapparent to skilled artisans that a particular method of compacting thepowder is not critical. The terms "cold compaction" and the like referto compression of the particulate mixture at a temperature below glasstransition or decomposition temperature of the organic binder. The greenpreform can be produced by such methods as cold uniaxial pressing (StepC" in FIG. 1), cold isostatic pressing (Step C in FIG. 5), injectionmolding (Step C'" in FIG. 1) or by processes such as cold extrusion andtape casting (not shown in FIG. 5). The particulate mixture ispreferably subjected to uniform compacting forces in order to provide anunsintered blank which has a uniform density. Alternatively, nearnet-shape green blank of the compounds 154 and 156 (of materials feeder100) are generated using dry pressing and injection molding processesrespectively.

The particulate mixture of zirconia alloy and/or zirconia-aluminacomposite is compacted; heated to a temperature range at which sinteringwill occur; sintered, that is, maintained at that temperature range fora period of time; and then cooled. During all or part of sintering, theparticulate mixture compact or the "green preform" is in contact withdopant, as discussed below in detail. In FIG. 5, element 152 representsthe product of both mixing chemical species and binders and subsequentcompaction, indicated by arrows A, A', B & C', respectively. Compactionand sintering are generally discussed herein as two consecutiveoperations, as indicated by steps C and E, respectively, in FIG. 5,however, the invention is not limited to a particular sequence ofcompacting and sintering. For example, compaction and sintering can besimultaneous in a single operation or partial compaction can be followedby sintering and further compaction. The interim product of compactingand sintering operations is referred to herein as a "blank", which isillustrated as element 152 in FIG. 1. Blank 152 is at least partiallycompacted and is either unsintered or not fully sintered.

In a preferred method of the invention, the powder is cold compacted toprovide a "green preform", which has a "green" density that issubstantially less than the final sintered density of the ceramiccomponent 160 of materials feeder 100. It is preferred that the greendensity be between about 40 and about 65 percent of the final sintereddensity, or more preferably be about 60 percent of the final sintereddensity.

Referring to FIG. 6, using press 150, the cold isostatic pressing of thepowder was done by filling rubber mold 162 with powder 144 or 148 andkeeping the mold 162 sealed by plate 164 in autoclave 166 of coldisostatic press 150. Mold 162 is then pressurized to about 25,000 poundsper square inch. Seal plate 164 may either be a metallic material, suchas aluminum or steel or a hard rubber. Thus, in accordance with FIG. 5,Step D, near-net-shape components 158 of materials feeder 100 are formedby green machining (as discussed below) of the blank 152 using carbidetools. Then the green components are sintered to full density usingpreferably sintering schedules described in U.S. Pat. Nos. 5,336,282 and5,358,913, hereby incorporated hereby by reference, and final precisionmachining were made to tight tolerances to produce the components ofmaterials feeder of the invention using diamond tools. Near net-shapedgreen preforms 154 or 156 produced either by dry pressing or byinjection molding respectively, did not warrant green machining togenerate net-shaped components after sintering. The near-net-shapedgreen preform 156 produced by injection molding needed an additionalstep called "debinding" wherein excess organic binders are removed byheating the preforms at around 250° C. for about 12 hours prior tosintering.

Sintering

Once again referring to FIG. 5, Step E, sintering of the green machinedcomponents of materials feeder 100 is performed in a temperature rangefrom about 1400° C. to about 1600° C., or more preferably about 1500° C.Preferable sintering times is in the range from about 1 hour to about 3hours, or more preferably, about 2 hours. In a particular embodiment ofthe methods of the invention, the sintering peak temperature is 1500° C.and that temperature is maintained for about 2 hours. It is preferredthat the pre-sintered components of materials feeder 100 be slowlyheated to sintering temperature and slowly cooled so as to avoidundesirable dimensional changes, distortions and crack development. Inan embodiment of the invention having a preferred sintering temperatureof 1500° C., preferred temperature ramps during heating are: about 0.3°C./minute for room temperature to about 300° C., about 0.1° C./minutefor about 300° C. to about 400° C., about 0.4° C./minute for about 400°C. to about 600° C., and about 1.5° C./minute for about 600° C. to about1500° C. Preferred temperature ramps during cooling are: about 2°C./minute for about 1500° C. to about 800° C. and about 1.6° C./minutefor about 800° C. to room temperature.

Alternatively, sintering may be achieved in the presence of a dopantselected from: MgO, FeO, ZnO, NiO and MnO, and combination thereof, asdiscussed below in detail. The resulting zirconia-alumina ceramiccomposite components of the materials feeder of the invention has a coreof α-alumina and tetragonal zirconia and a case of cubic spinel or cubicspinel along with cubic structure or cubic and monoclinic or tetragonalstructure of zirconia alloy. For zirconia alloy ceramic, sintering inthe presence of dopant selected from "Mg-Ca-Y-Sc-rare earth oxides" willproduce articles with cores having tough tetragonal crystal structureand the cases having hard cubic crystal structure.

In the sintering process, the dopant oxide selected from MgO, FeC, ZnO,CoO, NiO, and MnO, and combination thereof, is in contact with theblank. It is preferred that the sintering result in a ceramic componentshaving a "full" or nearly theoretical density, and it is more preferredthat the density of the ceramic components be from about 99.5 to about99.9 percent of theoretical density. Sintering is conducted in air orother oxygen containing atmosphere.

The methods of the invention are not limited to any particular sinteringpressure and temperature conditions. Sintering can be performed atatmospheric pressure or alternatively a higher pressure, such as thatused in hot isostatic pressing can be used during all or part of thesintering to reduce porosity. The sintering is continued for asufficient time period for the case of the article being sintered toreach a thermodynamic equilibrium structure. An example of a usefulrange of elevated sintering pressures is from about 69 MPa to about 207MPa, or more preferably about 100 to 103 MPa.

The exact manner in which the dopant is in contact with the blank duringsintering is not critical, however, the "case" as that term is usedherein, is limited to those areas of the blank in contact with thedopant during sintering. For example, a cubic spinel and tetragonalzirconia case can be readily produced by the methods of the invention ona portion of the overall surface of an article. It is not critical thatthe dopant be in contact with the blank during initial sintering, thatis, sintering which does not result in an increase in density to fulldensity.

Prior to observing the results of the examples, the inventors hadthought that they would be able to provide an explanation for conversionmethods having any relative percentages of zirconia alloy and alumina.The inventors had expected results to be in accord with the conceptsthat the formation of cubic spinel is highly favored thermodynamicallyover the conversion of tetragonal zirconia to cubic zirconia and thatthe mechanism of action follows alumina concentration.

What has been discovered by the inventors is that, surprisingly, if theconcentration of alumina in the blank 152 is from about 5 weight percent(relative to the total weight of zirconia and alumina) to about 50weight percent, then the method of the invention produces an articlehaving a case that is cubic spinel and tetragonal zirconia and a corethat is a-alumina and tetragonal zirconia. During sintering, dopant, ineffect, diffuses past tetragonal zirconia until all of the dopant hascontacted and reacted i.e., "partitioned", with alumina. In contrast, ifthe concentration of alumina in the blank is less than about 5 weightpercent or greater than about 75 weight percent, then the method of theinvention produces an article that has a case that is substantiallycomposted of cubic spinel and cubic zirconia or cubic and monocliniczirconia and a core that is a-alumina and tetragonal zirconia. Duringsintering, dopant does not, in effect, diffuse past tetragonal zirconiauntil all of the dopant has contacted and reacted with alumina; butrather reacts with alumina and tetragonal zirconia in the same vicinity,leaving alumina deeper within the blank unreacted.

These results are not compatible with a simple mechanism of action basedon concentration alone. The results seen are compatible with a mechanismof action based upon an unpredictable alignment of several competingfactors, such as rate of diffusion of dopant during sintering.

Shaping/Machining

It is known that ceramic parts can be fabricated to net-shape by thecompaction processes such as dry pressing, injection molding, slipcasting, and extrusion or cold isostatic accompanied by green machining(FIG. 5, Step D). Green machining refers to the process of machining theceramic particulate compact prior to densification. (For more generalinformation refer to David W. Richardson, Modern Ceramic Engineering:Properties, Processes and Use in Design, 2nd Edition (1992)). In thisprocess, it is important that care be exercised to avoid overstressingthe fragile material and producing chips, cracks, breakage, or poorsurface. For instance, it is important that the ceramic preform is heldrigidly, but with no distortion or stress concentration, during greenmachining. The part can be rigidly held by one of a numerous ways,including by simple mechanical gripping, by bonding or potting with acombination of beeswax and precision metal fixtures, the latter beingpreferred by the inventors. Once the ceramic preform is secured rigidlyin a fixture, green machining can be accomplished in a variety ofmethods, including: turning, milling, drilling, form wheel grinding, andprofile grinding. The inventors prefer turning and profile grinding thepreform during green machining to achieve the best results. Machiningcan be either dry or wet, depending on the binder present and whether ornot the part has been bisque fired, i.e., fired at a high enoughtemperature to form bonds at particle-particle contact points, but notat a high enough temperature to produce densification.

Apart from green machining, a further precision machining Step F,according to FIG. 5, of some of the surfaces is required to meetdimensional tolerances and to achieve improved surface finish or removesurface flaws. Maintaining dimensional tolerances to the extent of fewmillionths of an inch or achieving surface finish to less than 10microinches is not possible unless final machining after sintering isundertaken. We accomplished dimensional tolerances to the extent of ±100millionth of an inch using dry pressing (uniaxial pressing) for partsfor simple geometry and controlling the shrinkage by our patentedsintering process. Precision machining is required when the part demandstolerances in certain dimensions less than 100 millionth of an inch andalso to meet some dimensional tolerances such as roundness,perpendicularity, parallelness, etc. As contrasted with green machining,final precision machining of the sintered ceramic bearing requiresdiamond tooling and also sophisticated machines. Skilled artisans wouldknow that milling, grinding, lapping and polishing are some effectiveprocedures which are used in precision machining.

This invention is further clarified through the following examples:

Working Example 1:

Zirconia ceramic powder (prealloyed with secondary oxides described inU.S. Pat. Nos. 5,336,282 and 5,358,913) were packed and sealed in moldsmade of rubber/polyurethane of 55 to 70 durometers of Shore hardness A(preferably 65 durometers). These molds were cold isostatically pressedin an autoclave at 15 to 30 kpsi (preferably 25 kilo pounds per squareinch) to obtain preforms of appropriate sizes in width and length.

Working Example 2:

Same as in working Example 1, except that the zirconia alloy powder ispremixed with polyvinyl alcohol binder.

Working Example 3:

Same as in working Example 1, except that the zirconia alloy powder ispremixed with acrylic binder.

Working Example 4:

Same as in working Example 1, except that the ceramic material is acomposite of particulate zirconia alloy and particulate alumina ofvarying amount of from 5 to 50 weight percent. The binders used in theseworking examples are the same as that of in the working examples 2 and3.

Working Example 5:

Preformed blanks produced by cold isostatically pressing are machined intheir green state (i.e., before sintering) to produce near-net-shapearticles using carbide tools in lathe and milling machines. The cuttingspeeds in green machining of zirconia preforms were maintained between2800 and 3400 rpm (preferably at 32,000 rpm).

Working Example 6:

Near net shaped articles of working example 5 were also produced by drypressing using a die-punch assembly and a cold compacting machine.

Working Example 7:

The green machined near-net-shaped components of the materials feedermade by cold isostatically pressing or near-net-shaped feeder assemblagemade by cold pressing are sintered following schedule described in U.S.Pat. Nos. 5,336,282 and 5,358,913. After sintering the ceramiccomponents of the materials feeder equipment achieved full theoreticaldensity of 6.05 gms/cc for yttria stabilized zirconia.

Working Example 8:

The final precision machining of the components of the materials feederand lapping of the reciprocating motion path were carried out usingdiamond tools. The surface finish of the feeder assemblage was <0.1microns.

Working Example 9:

The components of the materials feeder were retrofitted in a powdercompacting machine. The powder used for compacting in this machine wasof an intermetallic magnetic alloy, more particularly, an alloy ofneodymium-boron-iron. This alloy powder is extremely hard and brittle.The components of the materials feeder of this invention survived inexcess of ten million cycles of reciprocating motion for powder feed inthe die for subsequent compaction.

Comparative Example:

The prior art components of the materials feeder for the compactingmachine described in the above working example is made of hardenedsteels. When the magnetic alloy powder described in the above workingexample was used in such machines, the components of the materialsfeeder did not survive in operation even up to a million cycle.

While the invention has been described with a certain degree ofparticularity, it is manifest that many changes may be made in thedetails of the construction and the arrangement of components withoutdeparting from the spirit and scope of the disclosure. It is understoodthat the invention is not limited to the embodiments set forth hereinfor purposes of exemplification, but is to be limited only by the scopeof the attached claim or claims, including the full range of equivalencyto which each element thereof is entitled.

Parts list

The invention has been described using the following reference numeralsto identify the parts and features disclosed:

100 . . . materials feeder equipment

114 . . . feeder box

120 . . . base or first sliding portion of feeder box 114

124 . . . sidewalls

126 . . . ware plate

128 . . . second sliding portion of ware plate 126

130 . . . pneumatically controlled ram

132 . . . die cavity

134 . . . retaining ring

136 . . . die punch

140 . . . zirconium oxide or zirconia powder

142 . . . secondary oxide powders

144 . . . zirconia alloy powder

146 . . . aluminum oxide or alumina

148 . . . particulate mixture of Y-TZP and alumina or zirconia-aluminacomposite

150 . . . isostatic press

152 . . . unsintered blank, or green preform, made by cold isostaticprocess

154 . . . unsintered blank, or green preforms, made by dry pressing

156 . . . unsintered blank, or green preforms, made by injection molding

158 . . . near net-shape components

162 . . . rubber mold

164 . . . seal plate for rubber mold

166 . . . autoclave

We claim:
 1. An improved wear and abrasion resistant ware plate usefulfor materials feeder equipment in which at least one element in slidingcontact with the ware plate has a tetragonal zirconia polycrystalceramic sliding surface, the improvement comprising the ware platehaving a zirconia-alumina ceramic composite sliding surface for slidingcontact with said zirconia-alumina ceramic composite sliding surfacebetween a first and second position.
 2. An improved wear and abrasionresistant ware plate useful for materials feeder equipment in which atleast one element in sliding contact with the ware plate has azirconia-alumina ceramic composite sliding surface, the improvementcomprising the ware plate having a tetragonal zirconia polycrystalceramic sliding surface for sliding contact with said zirconia-aluminaceramic composite sliding surface between a first and second position.